Method for fabricating battery cases

ABSTRACT

A molding method for injection molding long thin walls of uniform thickness such as the walls of a battery case. The uniform thickness walls are molded between cores. The pressure at the walls is relieved, preferably by withdrawing some of the cores, before the molded part is entirely separated from the mold. Dimensional stability is improved by rigidly supporting the mold cores during the initial stage of plastic injection.

United States Patent Halsall et al.

[4 1 June 27, 1972 METHOD FOR FABRICATING BATTERY CASES Vincent MichaelHalsall, Bayside, Wis.; Peter Anthony Cosme, Lockport, lll.

Assignee: Globe-Union Inc., Milwaukee, Wis.

Filed: Aug. 19, 1968 Appl. No.: 798,205

Related U.S. Application Data Division of Ser. No. 608,104, Jan. 9,1967, Pat. No. 3,509,603.

Inventors:

U.S. Cl ..264/328, 264/318, 264/334 Int. Cl. ..B29f 1/14 Field of Search..264/328, 334, 336, 318;

References Cited UNlTED STATES PATENTS Morin ..264/297 X l/l969 Spatz..264l3l8 X Lensky ..18/42 X 1,394,558 10/1921 Klug ..264/248 2,330,3699/1943 Marsh.... ....18/42 D 2,542,263 2/1951 Schultz ....264/328 X2,592,296 4/1952 Kutik ....264/328 X 3,243,752 3/1966 Lawrence....264/328 X 3,296,353 1/1967 Nouel ....264/328 X 3,402,713 9/1968Senkowski et al. ..264/3 34 X FOREIGN PATENTS OR APPLICATIONS 962,72612/1949 France ..264/328 Primary ExaminerRobert F. White AssistantExaminer-Richard R. Kucia AttorneyPendleton, Neuman, Seibold & Williams57 ABSTRACT A molding method for injection molding long thin walls ofuniform thickness such as the walls of a battery case. The uniformthickness walls are molded between cores. The pressure at the walls isrelieved, preferably by withdrawing some of the cores, before the moldedpart is entirely separated from the mold. Dimensional stability isimproved by rigidly supporting the mold cores during the initial stageof plastic injection.

2 Claims, 4 Drawing Figures Patented June 27, 1972 3 Sheets-Sheet 2 Fig.3.

INVENTORS V/NCE/VT M. HALSALL BY PETER A. COS/WE PEA/OLE TON, NEUMA/VSE/BOLD 8 WILLIAMS nrromvsrs METHOD FOR FABRICATING BA'I'I'ERY GASESThis application is a division of application Ser. No. 608,104 filedJan. 9, 1967, now U.S. Pat. No. 3,509,603, issued May 5, 1970.

Typical prior art storage battery cases are made of rubber compositionwith wall thicknesses generally in excess of 0.20 inch. Thick walledrubber cases are heavy and the relatively thick walls limit the volumeavailable for acid. Low acid volume results in low battery capacity. Itis therefore desirable to fabricate battery cases with the thinnersections of the walls less than 0.10 inch from a lightweight,thermoplastic such as polypropylene.

One of the most economical methods for producing thermoplastic parts isinjection molding. Briefly, this process involves injecting athermoplastic material heated to a liquid state into a multi-piece mold.The mold defines the configuration of the part to be manufactured. Thethermoplastic material hardens, the mold pieces separate, and the partis ejected.

Injection molding of relatively large, thin-walled battery cases posessome unique problems. To facilitate separation of mold pieces in aconventional injection molding process, long thin walls of the moldedpart are made with a rather appreciable draft or taper. However, theside and wall portions of a battery case, especially a thin-walledbattery case, are preferably of approximately uniform thicknessthroughout. Unlike tapered walls, uniform thickness walls have no thickportions to reduce acid volume, and no thin portions of high stressconcentration. Attempted fabrication of uniform thickness walls byconventional molding techniques has resulted in the walls being tornfrom the rest of the part as the mold pieces are separated.

Furthermore, since the battery walls are relatively long the mold mustcontain a plurality of relatively long cores. Long cores tend to move asplastic is injected into the mold thus preventing accuracy anduniformity in wall thickness.

It is therefore an object of the present invention to provide a novelmethod and apparatus for producing battery cases.

It is a further object of the present invention to provide a novelinjection molding technique for fabricating thin wall battery cases.

It is a further object of the present invention to provide a novel moldfor injection molding thin wall battery cases.

It is a further object of the present invention to provide a novelmethod and apparatus for injection molding long thin walls of uniformthickness.

Briefly, the above objects may be accomplished in an injection moldingapparatus which performs one or more of three functions: (1) during aninitial plastic filing period the mold cores are rigidly secured at twoends, but after the initial period the support at one end of the coresis withdrawn permitting complete filling of the cavity; (2) after aninitial setting period and before the part is entirely separated fromthe cores, the pressure at the walls of the part is relieved and; (3)the part is removed from the stationary part of the mold before it isentirely hardened.

In a preferred embodiment, the first of these functions is accomplishedby securing each core at one end in a conventional manner and at itsother end to the stationary part of the mold by a plurality of movablepins. While securing the cores, the movable pins occupy space to befilled with plastic. While the plastic is being injected into the mold,and before the plastic hardens, the pins are removed from the cores adistance so that they are flush with the remainder of the stationaryportion of the mold. The plastic continues to flow and fill the holesleft by the pins.

The second function mentioned above is accomplished in preferredembodiment of the invention by separating the part from the cores in twosteps. First, some of the cores are removed to relieve pressure at thewalls. Next, the part is separated from the remaining cores.

The third function is performed by removing the part from that portionof the mold corresponding to the outer face of the battery case beforethe part is separated from the alternate cores.

A more detailed description of the present invention will be given withreference to the accompanying drawings in which:

FIG. 1 is an isometric view of a storage battery case, and

FIGS. 2 and 3 are sectional top and side views, respectively, of aninjection mold constructed according to the present invention.

FIG. 4 is a sectional top view showing the mold in an intermediate stageof the operation.

FIG. 1 illustrates a battery case 10 which might be used, for example,in the manufacture of an automobile storage battery. The case includesfour side portions 11, a bottom portion 12, and a plurality of interiorwall portions 13. Generally, at least one outer side wall of the batteryhas a raised portion 16 including lettering or merely a texturedsurface. The side and wall portions form six cavities 15 which house sixcorresponding battery cells (not shown) when the battery is completed. Acover portion (not shown) is fitted over the top of the cavities 15 toform the complete battery.

Each of the six cavities 15 of a completed battery contains a sulphuricacid electrolyte. The volume of battery acid, which is a primary factordetermining battery capacity, is limited by the volume occupied by thebattery cells and by the walls forming the cavities. For a battery offixed overall dimensions, reduction in wall thickness results in anappreciable increase in the volume of acid contained within the battery.It is therefore advantageous to fabricate the battery case from athermoplastic material such as polypropylene which enables constructionof walls less than 0.10 inch thick with a substantial increase in acidvolume and battery capacity. Additional advantages of polypropylene areits light weight and strength. The optimum dimensions for the casedepend, of course, on its use. For normal automotive purposes, the wallsmay have an average thickness of about 0.085 inch. The bottom portionmay be slightly thicker, e.g., about 0.095 inch. The overall dimensionsfor such a case might, for example, be 6 inches X 8 inches X 10 inches.

As mentioned above, one of the most economical methods of producingthermoplastic parts is injection molding. If the battery case shown inFIG. 1, however, were manufactured along conventional injection moldingtechniques the side and wall portions ll, 13 would have to beappreciably tapered to be thickest at the bottom and thinnest at the topof the case. A taper, or draft, of approximately 0.025 to 0.030 inch perinch is necessary in conventional molding in order to facilitate theremoval of the mold cores, which fonn the six cavities, 15, withoutdamage to the molded part. When one attempts to pull cores inconventional systems having little or no taper then the cores tend totear the wall portions from the rest of the case.

On the other hand, tapered walls are not entirely consistent with theadvantages of thermoplastic cases set out above. With tapered wallseither strength of the battery case or acid volume capacity has to besacrificed. A sufi'rciently strong battery case must have a minimum wallthickness which would correspond to the thinnest portion of the taperedwall. The remainder of the wall would have to have an increasedthickness due to the taper. The increased thickness requires extramaterial and decreases acid capacity.

A mold constructed according to the present invention, however, permitsfabrication of walls having approximately uniform thickness. Anegligible taper on the order of 0.025 inch per inch may advantageouslybe employed without the disadvantages of the large conventional taper.

FIGS. 2 and 3 are cross sectional views of an injection mold forfabricating battery cases according to the present invention. Theillustration of FIGS. 2 and 3 is intended to set forth the novelfeatures of one embodiment of the present invention. Some features whichare conventional in injection mold construction have not beenillustrated for the sake of simplicity of description. For example, allthe mold parts advantageously include conventional cooling channelswhich are not illustrated.

The mold includes a first part 20, generally temied a stationary part,which is rigidly secured to a conventional injection molding machine(not shown). A flat, relatively rectangular, face 21 of the stationarypart 20 forms the bottom portion of a mold cavity in the shape of thebattery case.

The stationary part 20 includes a plurality of runners 22 connected toconventional injection molding heads 23, including heating elements 2.4,which in turn are connected through additional channels 25 to aninjection nozzle 26. The nozzle 26 is connected to the plastic injectionportion of the molding machine (not shown). Plastic injected from themachine travels through the nozzle 26 to the runners 22 and eventuallyto the portion of the mold formed by the face 21. The design of theplastic injection system follows conventional practices of the moldingart.

The stationary part 20 also contains twelve bores 30 containing pins 31mounted for axial movement therein. The pins 31 are connected on one endto a plate 32 which is movable to and from the stationary part 20. Attheir other ends, the pins 31 project into recesses 33 formed in aplurality of movable cores 40, 41. Stop members 47 are provided to limitthe movement of the plate 32 so that the pins 31 can be moved back justfar enough to be flush with the face 21.

The cores 40, 41 form the inside of the bottom portion 42 and theinterior wall portions 43 of the mold cavity. Near the bottom 42 of themold cavity a plurality of channels 44 are formed in the mold cores 40,41. The channels 44 form a plurality of projections 45 (FIG. 3) whichsupport the battery plates in the completed battery.

The outer side portions 50 of the mold cavity are fonned by four members51 which are slidably mounted on rods 52 rigidly connected to thestationary part 20 of the mold. Each rod 52 carries a spring 53connected to both the rod 52 and the slidable part 51. The spring 53tends to urge the slidable part 51 away from the stationary part 20. Oneor all of the four faces of the four slidable parts 51 will generally beraised. For example, one or more of the four faces may fonn a figure 16such as that shown on the case in FIG. 1.

The first three movable cores 40 are rigidly secured, and preferablyformed integrally with, a large plate 60 which rests against a stripperplate 61. The other three cores 41 are formed integrally with a numberof projections 62 which are secured to a second plate 64 by suitablefasteners 65. A plurality of wear strips 66 are positioned in theprojections 62 to provide a surface along which the engagement portions63 of the first plate 60 slide. Attached to the plate member 64 are twohydraulic cylinders 67 of conventional design. The two movable plungers68 of the two hydraulic cylinders 67 are connected to the stripper plate61. When the two cylinders 67 are actuated, the plate 64 supporting thecores 41 separates from the stripper plate 61. Each of the first threecores 40 includes two air passages 46 for contacting air hoses 47 whichmay assist in part ejection.

Holes 70 are formed in the second core plate 64 to permit guide pins 71attached to the first core plate 60 to pass therethrough. Also, two stopmembers 72 are provided to limit the motion of the first core plate 60,when, as explained below, it moves toward the second core plate 64.

The stripper plate 61 carries four engagement members 80 which haveprojections 81 adapted to engage complementing projections 82 on thefour slidable members 51. The interface 83 between the projections 81,82 is set at an angle which allows the members 81, 82 to slide relativeto one another in an operation where the members 80 pull the slidablemembers 51 away from the stationary part 20 of the mold.

A recess 85 is formed in each of the four slidably mounted members 51 topermit a lip portion 86 to be formed on the upper end of the batterycase. One side of the lip portion 86 engages the stripper plate 61.

A drive plate 90 is rigidly connected to the first core plate 60 bysuitable members 91. A drive mechanism in the molding machine isconnected to the drive plate 90 in the conventional manner.

With the mold in its closed position, as shown in the drawing, plasticin a liquid state is injected into the mold cavity in a conventionalmanner. It is desirable that the plastic be a high tensile strength,high impact strength, mar resistant, thermoplastic hydrocarbon. Forexample, a polypropylene or modified copolymer of polypropylene havingthe following characteristics has been found suitable:

l. aspecific gravity of0.85 to 1.05;

2. a tensile yield strength (at 2 inches per minute on a :4; inch thickspecimen) of approximately 4,000 p.s.i. when tested in accordance withASTM test method D638-61T;

3. a modulus in flexure (at 0.05 inches/minute on a V8 inch thickspecimen) of approximately 180,000 p.s.i. when tested in accordance withASTM test method D790-63; and

4. a Rockwell hardness range of 63 to 95 when tested in accordance withASTM test method D785-60T.

Other suitable materials may be low pressure polyethylene and highimpact polystyrene.

The liquid plastic is injected through the nozzle 26 to the plurality ofrunners 22. The heating elements 24 maintain the plastic in a liquidstate. The l2 core supporting pins 31, which are positioned in recesses33 in the six cores 40, 41 hold the cores 40, 41 in a stationaryposition while the plastic is being injected into the mold. In this waythe flowing plastic cannot move the cores 40, 41 and a high degree ofdimensional stability is achieved. As soon as the cavity is completelyfilled with plastic, and while the plastic is still in a very liquidstate, the pin supporting plate 32 is moved away from the stationarypart 20 and the pins 31 are removed from the cores 40, 41. The pins 31are moved back a distance so that their ends are flush with the face 21of the cavity fonned by the stationary part of the mold. Liquid plasticthen fills the spaces in the cavity previously occupied by the pins 31.Since the plastic supplying channels 22, 25 are not closed to thecavity, the additional material necessary to fill the pin spaces issupplied through the injection system.

After an initial period of hardening, e.g., about 30 seconds for a partof the dimensions here involved, during which water or other coolingfluid is circulated through all the mold parts in a conventional manner,the drive plate 90 of the mold, which is connected to the movable platenof the injection machine (not shown) is moved away from the mold causingremoval of the first three mold cores 40 from the cavity. This positionof the mold is shown in FIG. 4. Removal of only the first three cores 40relieves the pressure on the wall portions of the part. Pressure isrelieved not only at the walls immediately adjacent the removed cores,but also at the one outer wall which is not adjacent to a removed core.It is not generally necessary to remove the core adjacent a wall torelieve pressure at the wall although it is convenient to do so.

As the drive plate 90 continues moving, the first core plate 60 engagesthe stop members 72. The remaining three cores 41 and the stripper plate61 then move away from the mold. The stripper plate 61 carries the fourslidably mounted portions 51 away from the mold. The plastic mold partis removed .from the cavity because it grabs the three remaining cores41.

As the four slidably mounted members 51 move along the rods 52 out ofthe stationary part 20 of the mold, the faces of these members 51, whichform the four outer side portions of the battery case, moveperpendicularly relative to the side portion. This perpendicular motionprevents damage to any raised portion on the outer part of the batterycase.

When the plastic part is completely hardened, which requiresapproximately another or seconds, the two hydraulic cylinders 67 areactuated and the stripper plate 61 removes the case from the remainingcores 41. The part then falls out of the mold and the mold is ready toform another battery case. The drive plate 90 is moved to close the moldand the process is repeated.

While the invention has been described with respect to a preferredembodiment, it should be noted that the invention is not limited to thepreferred embodiment. The scope of the invention is defined by thefollowing claims.

What is claimed is:

l. The process of injection molding a battery container having a bottomportion, a plurality of side portions, and a plurality of long thininterior wall portions, said interior wall portions being parallel toone another and to two of said side portions, the thickness of the majorportion of said walls being no greater than about 0.10 inch and having ataper of no more than about 00025 inch per inch, comprising the stepsof:

a. providing a mold including a first section for forming the outerportion of said container, including said side and bottom portions, anda plurality of cores positioned with respect to said first section toform the inner portions of said side and bottom portions and theinterior walls of said container,

b. maintaining a rigid connection between said cores and said firstsections at the portion of said cores corresponding to said bottomportions,

c. injecting a thermoplastic material into said mold and disestablishingsaid rigid connection before said thermoplastic material hardens,

. removing only alternate ones of said cores from said mold subsequentto plastic injection and after an initial period of hardening,

e. separating the remaining cores from said first section whereby saidplastic remains attached to said cores and separates from said firstportion, and

f. removing the plastic from said cores.

2. The process of injection molding a battery container having a bottomportion, a plurality of side portions, and a plurality of long thininterior wall portions, said interior wall portions being parallel toone another and to two of said side portions, the thickness of the majorportion of said walls being no greater than about 0.10 inch and having ataper of no more than about 0.0025 inch per inch, comprising the stepsof:

a. providing a mold including a first section for forming the outerportion of said container, including said side and bottom portions, anda plurality of cores positioned with respect to said first section toform the inner portions of said side and bottom portions and theinterior walls of said container,

b. injecting a thermoplastic material into said mold,

c. removing only alternate ones of said cores from said mold subsequentto plastic injection and after an initial period of hardening,

d. separating the remaining cores from said first section whereby saidplastic remains attached to said cores and separates from said portion,and

e. removing the plastic from said cores.

UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION Patent No. 3{673 Dated June 27 1972 Inventor) Vincent Michael'I-Ialsall' and PeterAnthony Cosme It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 2,' line 63, "0.025" should be .0025

Column 5, line 15, (Claim 1) delete "said" before "first" Column 6,line'22, (Claim 2) insert first before "portion" Signed and sealed this15th day of May 1973.

(SEAL) Attest:

EDWARD M.F LETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PQ'WSQ USCO MM-DC wave-Pea U.5, GOVERNMENT PRINTINGOFFICE: ,l

1. The process of injection molding a battery container having a bottomportion, a plurality of side portions, and a plurality of long thininterior wall portions, said interior wall portions being parallel toone another and to two of said side portions, the thickness of the majorportion of said walls being no greater than about 0.10 inch and having ataper of no more than about 0.0025 inch per inch, comprising the stepsof: a. providing a mold including a first section for forming the outerportion of said container, including said side and bottom portions, anda plurality of cores positioned with respect to said first section toform the inner portions of said side and bottom portions and theinterior walls of said container, b. maintaining a rigid connectionbetween said cores and said first sections at the portion of said corescorresponding to said bottom portions, c. injecting a thermoplasticmaterial into said mold and disestablishing said rigid connection beforesaid thermoplastic material hardens, d. removing only alternate ones ofsaid cores from said mold subsequent to plastic injection and after aninitial period of hardening, e. separating the remaining cores from saidfirst section whereby said plastic remains attached to said cores andseparates from said first portion, and f. removing the plastic from saidcores.
 2. The process of injection molding a battery container haVing abottom portion, a plurality of side portions, and a plurality of longthin interior wall portions, said interior wall portions being parallelto one another and to two of said side portions, the thickness of themajor portion of said walls being no greater than about 0.10 inch andhaving a taper of no more than about 0.0025 inch per inch, comprisingthe steps of: a. providing a mold including a first section for formingthe outer portion of said container, including said side and bottomportions, and a plurality of cores positioned with respect to said firstsection to form the inner portions of said side and bottom portions andthe interior walls of said container, b. injecting a thermoplasticmaterial into said mold, c. removing only alternate ones of said coresfrom said mold subsequent to plastic injection and after an initialperiod of hardening, d. separating the remaining cores from said firstsection whereby said plastic remains attached to said cores andseparates from said portion, and e. removing the plastic from saidcores.